As known in the art, microprismatic retroreflective film (also known as retroreflective sheeting or reflective film) generally consists of a plastic film containing many microscopic cube corner retroreflective elements (also known as either microprismatic retroreflective elements or microprisms). These retroreflective elements have three mutually substantially perpendicular lateral faces, which intersect at a single point, or apex. These cube corner retroreflective elements operate to return impinging light towards its source. Light enters each cube corner retroreflective element and is then reflected from each of the three lateral faces to return towards its source. Such microprisms are generally shaped like a tetrahedron, but also exist in truncated versions, known in the art as full-cube microprisms.
Reflection from the three lateral faces occurs either through specular reflection or total internal reflection. With specular reflection, the cube corner retroreflective elements are coated with a reflective material, such as either aluminum or silver, as is the case with metalized microprismatic retroreflective film. With total internal reflection, the cube corner retroreflective elements have not been coated with a reflective material, but instead are governed by Snell's Law where any light impinging on one of the lateral faces passes through the face unless it strikes the face at an angle less than its critical angle, in which case the light is reflected. Encapsulated microprismatic reflective film is one such retroreflective film construction where the microprisms operate through the principles of total internal reflection. Regardless of whether the microprisms function through total internal reflection or specular reflection, the tolerances on the microprisms must be tightly controlled to ensure that the lateral faces are substantially perpendicular to each other. Even minor deviations in the dihedral angles between the lateral faces from 90° can cause a substantial change in the retroreflective properties. Both encapsulated microprismatic reflective films and metalized microprismatic reflective films are commonly supplied with an adhesive backing to allow for application on sign faces or other substrates.
A cross-sectional diagram of the well-known microprismatic retroreflective film structure is shown in FIG. 1. Microprismatic retroreflective film 10 is made from a light-transmissive polymeric material and consists of a smooth outer-surface 11 and microprismatic retroreflective cube corner elements 12. Light impinging on the outer-surface 11 passes through the film to be reflected by the lateral faces 13 of the microprisms 12 and returned towards the source of the light as depicted by arrow 14.
Although FIG. 1 depicts the retroreflective film as a single layer of polymeric material, in practice most microprismatic reflective sheeting materials available on the market today consist of two or more layers of polymeric materials. For example, FIG. 2 shows the cross-section of a microprismatic retroreflective film 20 with two different polymeric layers. The first polymeric layer is known as the prism layer 25, which is a light-transmissive polymeric layer containing the microprisms 12. The second polymeric layer is the body layer 28. In this example, the outer surface 21 is part of the body layer 28, and the body layer 28 also functions as the outer surface layer of the retroreflective film construction. Although the prism layer 25 is made from a single layer of polymeric material, it can further be categorized into two different sections. The portion of the prism layer 25 above the base of the microprisms (as depicted by dashed line 29) can be defined as the land section 26 of the prism layer 25. The portion of the prism layer 25 consisting of the microprisms 12 can be defined as the prism section 27 of the of the prism layer 25. As such, the height of the microprisms 12 equals the thickness of prism section 27.
The reason for multiple layers in certain retroreflective film constructions is that each layer performs a different function to balance end-use performance and application properties against manufacturing considerations and raw material costs. For example, it may be desirable for a body layer to provide ultraviolet (UV)-light screening functions to enhance the overall durability and weatherability of the microprismatic sheeting. Such UV-light screening layers may protect underlying polymeric layers, any pigments or colorants, or any printed graphics or other printed images that may be printed on a lower layer. For example, Pavelka, et al., U.S. Pat. No. 5,387,458 outlines the use of protective UV-light screening layers to protect fluorescent colorants in a lower layer. As another example, to create a flexible microprismatic retroreflective construction, a two-layer construction is commonly used where the body layer is made from a flexible polymeric material and the prism layer is made from a rigid polymeric material. Such microprismatic constructions are discussed in Smith, et al., U.S. Pat. No. 5,450,235.
Generally speaking, it is preferable for the prism layer to be made from a rigid polymeric material, such as acrylic, polyester, or polycarbonate. This ensures that the precise dimensions of the microprisms can be maintained to maximize levels of retroreflection. If the microprisms were formed from a soft or flexible polymer, such as flexible polyurethane or plasticized polyvinyl chloride, the shape of the microprisms could be easily distorted and the levels of retroreflection could be greatly diminished.
One advantageous material to use as the prism layer is acrylic, such as polymethyl methacrylate acrylic. There are several reasons for this. First, it has lower processing temperatures (compared with other rigid polymers such as polycarbonate or polyester) and the microprisms can therefore be more easily formed into the prism layer. Further, compared to other polymeric materials such as polycarbonate, acrylic is less hydroscopic in nature and therefore less prone to generating moisture bubbles or similar defects during the molding or forming of the microprisms into the prism layer. Further, acrylic is generally a weatherable and durable polymeric material. Further, it appears to metalize more easily compared with other polymeric materials to provide a brighter metallic finish when used in metalized microprismatic retroreflective sheeting materials.
For microprismatic sheeting expected to last for an extended time frame in outdoor environments, it is also preferred to use acrylic polymeric materials, such as polymethyl methacrylate, as the outer surface layer. As mentioned above, acrylic polymers are naturally weatherable. The use of acrylic materials as the outer surface layer of the reflective film can prolong outdoor life of the film since acrylic polymers generally do not yellow, chalk, or haze over time as rapidly as do other polymeric materials. Such materials can further provide a UV-light screening functionality by blending UV-light absorbing additives (such benzophenone or benzotriazole additives) into this outer surface acrylic layer. Further, compared with other durable polymers, such as polyvinylidene fluoride polymers, acrylic is often more cost effective.
However, there are downsides to the use of acrylic materials. Acrylics can be relatively brittle compared to other polymeric materials. This can be true even when the acrylic polymers are impact-modified acrylic polymers. Microprismatic sheeting made from only acrylic polymers can easily crack upon impact or can easily snap or break when flexed. In some instances, the relatively brittle nature of acrylic polymers may create problems during application of the reflective sheeting. For example, if a strip of adhesive-backed reflective film is misaligned during application, microprismatic reflective film made from only acrylic polymers may crack or tear when being repositioned to correct the misalignment.
The relatively brittle nature of many acrylic materials can be further characterized by the Notched Izod impact strength of the polymers as measured by ASTM D256 at 23° C. For example, a typical polymethyl methacrylate acrylic polymer has a Notched Izod impact strength of about 15-20 J/m. Moreover, many impact-modified polymethyl methacrylate acrylic polymers still have a Notched Izod impact strength only up to about 60 to 70 J/m.
To solve this issue, conventional processes have attempted to substitute other polymeric materials for either the prism layer or the outer surface layer of the product. For example, others have substituted either polycarbonate or polyester polymeric materials as the prism layer. This may solve the brittleness issue as polymers such as polycarbonate are quite strong and impact resistant relative to acrylic. However, as referenced above, such polymers are less desirable to use as the prism layer due to processing considerations or deficiencies in metalizing the retroreflective film.
Others have attempted to solve this issue by incorporating a thin supporting film into the adhesive layer of adhesive-backed reflective films. For example, the retroreflective microprismatic body of the Avery Dennison V-5720 Conspicuity Tape product is believed to be made from only acrylic polymers. However, to solve this brittleness issue, the adhesive supplied with the product contains a thin polyester supporting film in the middle of the adhesive. Unfortunately, the retroreflective microprismatic body of this product is still prone to cracking and breaking, and such adhesive systems incorporating a supporting film can be more expensive to manufacture.